Electron Microscopy

Electron Microscopy

CAPPA employs Electron Microscopy for carrying out high resolution imaging to aid photonic device understanding and development as well as for product and process analysis for a wide variety of samples. A Scanning Electron Microscope [SEM] is analogous to an optical microscope but instead of illuminating the sample with light it images the sample surface by raster scanning it with a high-energy beam of electrons. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample’s surface topography as well as composition.

Environmental SEM

The centre’s Hitachi S-3700 Microscope is a Variable Pressure or Environmental SEM [ESEM]. This allows the system to perform low vacuum imaging (6 to 270 Pa) which enables observation of non-conductive samples like electronic components, and water containing samples such as cultured cells, without any sample preparation. In addition the S-3700N has an extremely large chamber which is able to accommodate specimens up to 300mm in diameter and 110mm in height (approx 12×4 inches). For smaller samples the chamber size facilitates large stage traverse meaning that any area on the specimen can be imaged without the need to remove and remount. Magnification is up to x300,000 with a resolution of 3-4nm depending on the vacuum and detector. The system has both a Secondary Electron [SE] Detector which provides topographical information and a Back Scatter Electron [BSE] detector which gives both elemental and topographic information. In addition there is an Environmental Secondary Electron Detector [ESED] which can be used in Variable Pressure mode as a standard SE detector has reduced performance in low vacuum.

The three images below show the flexibility of the system. The top image is of a conducting sample while the bottom two are both non-conducting. Each sample is imaged in its native form with no sample preparation or additional conductive coating. The Gold particles [Top], as expected for a typical conductive sample, can be imaged with the SE detector at high vacuum leading to the highest resolution [3nm]. The rubber adhesive sample [Middle] can be imaged successfully without any coating. The bottom image is the most challenging sample shown and is beyond the capabilities of standard SEMs. The sample is a daisy flower which is fully hydrated and imaged at x1k magnification to show the pollen grains and structure. Through control of the chamber pressure as well as the stage temperature the sample can be kept hydrated during imaging to preserve the natural physical structure and composition. Additionally through control or pressure and/or temperature the sample can be dehydrated and the physical effects studied and understood.

Conductive Sample
This is a Secondary Electron image of gold particles with a resolution of 3nm.

Non-Conducting Sample
This is a Back Scatter Electron image of a rubber adhesive pad successfully imaged without surface charging, despite having no conductive coat applied.

Biological Sample
This is a Back Scatter Electron image of a daisy flower under low vacuum conditions showing the pollen structure. Such images cannot be achieved in a standard SEM.

3-D Reconstruction

In an SEM the amount of backscattered electrons and their direction varies with the composition and topography of a specimen. CAPPA’s system has a multi-segment BSE detector which in conjunction with image reconstruction software can create 3-D profiles without movement or tilt of the sample. Below are images showing the reconstruction of the 3-D profile of the rubber sample shown above in the previous section. Each individual segment of the detector can be individually addressed to give images from different aspects without moving the sample. The depth and contours of the surface crater have been imaged in turn by each BSE segment to produce four different viewpoints [Middle]. The software can then reconstruct the 3-D profile based on the different segment images [Bottom].

Full BSE Image
This is the same BSE image as shown above. The image is collected with all segments of the detector being addressed.

BSE Segments
Each segment of the BSE detector is addressed singly to produce an image. This results in four different images each from a different aspect. As can be seen each image has a different shadowing.

3-D Reconstruction
Taking each individual segment image the software can use the different viewpoints to reconstruct the contours of the surface. This results in a 3-D profile of the surface which can be easily manipulated.